1. Field of the Invention
The present invention relates to an image forming apparatus, used in, for example, a laser printer or a digital copying machine, that forms an image as a result of performing charging, exposure, and development on a photosensitive member.
2. Description of the Related Art
An image forming apparatus includes a charging device which uniformly charges a photosensitive surface of a photosensitive drum, a latent image forming device which forms an electrostatic latent image, which is in accordance with recording information, on the charged photosensitive surface, a developing device which develops the electrostatic latent image, and a transfer device which transfers developing agent on the photosensitive surface onto a recording sheet. Further, the image forming apparatus generally includes a fixing device that fixes the developing agent on the recording sheet to the recording sheet.
In the image forming apparatus, an electrostatic latent image is sequentially formed while moving the photosensitive surface, the developing device selectively develops the electrostatic latent image with the developing agent, and the transfer device transfers the developing agent onto the recording sheet. The recording sheet having the developing agent transferred thereon is heated while being pressed by the fixing device, so that the developing agent is melted and fixed to the recording sheet.
When an electrophotography image forming apparatus is used for forming an electrostatic latent image, which is in accordance with the recording information, onto the charged photosensitive surface, a method of irradiation using laser light is generally used. The photosensitive surface includes a photosensitive drum and a photosensitive belt. When forming a latent image while moving the photosensitive surface, what is called a raster scanning method is generally used. This is a method in which laser is formed into a beam to scan and expose the photosensitive surface by an optical system.
Hitherto, the optical system of a scanning exposure system has generally used an Under Field Scanner (UFS) method (refer to FIG. 2) as a method of scanning the drum surface (image bearing member) with laser. However, to meet the demand of further increasing speed, an optical system using an Over Field Scanner (OPS) method (refer to FIG. 3) as a method of scanning the drum surface at a higher speed than in the UFS method is beginning to be used. These two methods differ as follows. The UFS method is a method in which a light beam that is smaller than a reflecting surface of a polygonal mirror 6A is used for irradiation, whereas the OFS method is a method in which a light beam that is larger than a reflecting surface of a polygonal mirror 6B is used for irradiation. The OFS method was born as a result of considering the following methods of increasing the scanning speed:
(1) increasing the number of scanning lines provided in one rotation as a result of increasing the number of surfaces of the polygonal mirror, and
(2) increasing the number of rotations of the polygonal mirror (that is, reducing the size of the polygonal mirror).
Due to such a structure, the OFS method is advantageous compared to the UFS method from the viewpoints of noise, number of rotations, heat generation, and startup speed. However, it has a problem in that light quantity distribution in a main scanning direction is not uniform.
The ununiformity of the light quantity distribution in the main scanning direction is caused by a change in the quantity of reflection light resulting from a change in the angle of the reflecting surface of the polygonal mirror as shown in FIG. 4.
Laser light emitted from a laser diode has a light intensity distribution characteristic that is not uniform, that is, what is called a Far Field Pattern (FFP) characteristic. When a light beam that is wider than the width of the reflecting surface of the polygonal mirror is incident upon the polygonal mirror, areas of the light beam having different light quantity distributions are reflected due to angles of the reflecting surface of the polygonal mirror as shown in FIG. 4. Therefore, the reflection light quantity in one-scanning period in a main scanning operation varies due to a distribution ununiformity caused by the FFP characteristic.
Due to the change in the reflection light quantity, the light quantity near a main-scanning direction end portion, where the angle of the reflecting surface is large, is less than that near a main-scanning direction central portion, where the angle of the reflecting surface is small. Therefore, when an image is formed, as shown in FIG. 5, the density at the main-scanning direction end portion is reduced. There is a demand for achieving high image quality in addition to increasing the speed of the image forming apparatus. Therefore, it is necessary to correct the density change to form a uniform image without any density change.
Further, the image forming apparatus is required, not only to achieve higher speed and provide higher image quality, but also to have a long life (that is, to be highly durable). To meet such a requirement, a highly durable amorphous silicon drum is beginning to be used as a photosensitive drum required to form an image. The amorphous silicon drum has high durability due to the number of durable drums being approximately 3 million compared to approximately 80 thousand for related OPC (organic semiconductor) drums. The amorphous silicon drum is already practically used in a black-and-white copying machine.
However, the amorphous silicon drum has manufacturing problems, that is, sensitivity ununiformity due to variations in the thickness of a photosensitive film. The sensitivity ununiformity influences charging and exposure, and occurs as density ununiformity of an image. The demand for higher image quality in recent years has given rise to the problem that the amorphous silicon drum cannot be allowed on the market. Consequently, it is necessary to correct the density ununiformity.
A technology regarding density ununiformity is discussed in Japanese Patent Laid-Open No. 2005-70069. Here, a reduction in laser light quantity at the end portion in the main scanning direction in the aforementioned OFS optical system is corrected. In addition, in particular, correction values related to a density change occurring when the resolution is changed are stored in a storing unit, and image data and various correction values corresponding to image coordinates are integrated to control emission intensity of laser light as correction data. By this, the image density is made uniform. To correct printing density in accordance with a density setting to further optimize the density, an image forming apparatus that corrects the density as a result of changing a γ curve is proposed (refer to Japanese Patent Laid-Open No. 2002-172817).
Regarding density ununiformity, a reduction in laser light quantity at the end portion in the main scanning direction in the aforementioned OFS optical system is corrected as a result of changing the laser light quantity in the main scanning direction. Two main methods are available as methods of changing the laser light quantity. In the first main method, the light quantity at a photosensitive drum surface is made uniform using optical components, such as a lens, a reflecting mirror, and an aperture. In the second main method, the light quantity at a photosensitive drum surface is made uniform as a result of electrically changing laser light emission current. Since, in the first main method, it is difficult to individually adjust correction values, the first main method is disadvantageous from the viewpoint of variations in characteristics of laser chips having different individual characteristics. The second main method is an example in which laser light emission is electrically controlled.
In the method of electrically controlling laser light emission, driving current is changed in one main-scanning period. Here, the emission light control is performed so that a central portion of an image has a small amount of driving current, and an image end portion where the light quantity is reduced has a large amount of driving current. Ordinarily, as shown in FIG. 6B, in the emission light control, correction is made using a constant driving current change curve in one main-scanning period. FIG. 6A shows a case in which the laser light driving current is constant.
It is known that the responsiveness of a laser diode differs due to a difference in the laser light driving current. More specifically, when the laser driving current is small due to, for example, a change in the responsiveness of a current control feedback system and in a differential efficiency, influenced by the driving current, the response speed of a laser chip is reduced. In addition, when an input pulse duty is the same, driving with a small driving current causes a laser light emission pulse to be thin, and/or causes the light emission pulse to be formed as if it is driven by a smaller driving current. Therefore, lighting times when the laser diode is turned on and off become different due to a difference in the driving current. Consequently, when the above-described controlling operation in which the driving current is changed in the main scanning period is carried out, the difference in the lighting times, caused by a difference in the driving current, becomes a change in the laser light quantity. Therefore, at the end portion and the central portion of the image in the main scanning direction, a difference between image densities occurs. Further, since the influence of this phenomenon is large when the laser lighting time is short, this phenomenon becomes a problem in, for example, a color printer or a copying machine, where a pulse width modulation is performed.
This problem will be described with reference to FIG. 9. To correct a reduced light quantity at an end portion in the OFS optical system, a correction control operation is carried out to increase and decrease the driving current in the one main scanning period as shown in FIG. 9. In this example, the correction control operation is carried out so that, in the main scanning direction, a central portion has 80% electric current, and the end portion has 100% electric current.
FIG. 10 shown next shows an emission light quantity linearity characteristic with respect to an input pulse duty in this case. This linearity characteristic is obtained by measuring a continuous output light quantity as a result of applying an input pulse corresponding to a pixel. The vertical axis represents a percentage of laser light quantity that is output with respect to a maximum laser light quantity for each electric current (corresponding to 80% and 100% in FIG. 10), in each input pulse duty, when driving is performed at each electric current. The horizontal axis represents a ratio between ON and OFF of an input pulse. The ratio between ON and OFF indicates what percentage of gradation with respect to a maximum gradation the input pulse corresponds to. For example, at a gradation of 100 in an 8-bit 256 gradation, the value at the horizontal axis is equal to 100/256*100(%). The values along the horizontal axis substantially correspond to the gradation of an input image. Therefore, the input pulse duty can be understood as indicating the gradation of image data.
From this graph, it can be understood that, when driving is performed at various driving currents and at the same input pulse duty, the ratios between an actually output laser light quantity and a laser light quantity to be normally output differ from each other. That is, for an input pulse duty providing an intermediate density (used, for example, when halftone printing is performed), the following is true. That is, even if an attempt is made to emit light at the same input pulse duty so as to print an image having uniform density, the ratio between the actually output laser light quantity and the laser light quantity to be normally output at the end portion, where the driving is performed at 100% electric current, differs from the ratio between the actually output laser light quantity and the laser light quantity to be normally output at the central portion, where driving is performed at 80% electric current. For example, when the input pulse duty for the driving at 100% electric current is 50, the laser light quantity that is 50% of the maximum laser light quantity for the driving at 100% electric current should be output. In addition, when the input pulse duty for the driving at 80% electric current is 50, the laser light quantity that is 50% of the maximum laser light quantity for the driving at 80% electric current should be output. However, according to the graph of FIG. 10, when the driving current is 80% driving current, and the input pulse duty is 50, the laser light quantity that is output is only approximately 40% of the maximum laser light quantity for the driving at 80% electric current.
In contrast, according to the characteristic graph, when the duty is 100%, the lighting is continuous. Therefore, the respective light quantities (that are not related to the responsiveness) differ by substantially an electric current ratio, but an ideal light quantity output (%) is achieved independently of the input pulse duty. Therefore, for example, the driving current corrections shown in FIG. 6 are properly reflected.
That is, the present invention can overcome the problem that, when the input pulse duty corresponds to an intermediate density (such as an input pulse duty of 50%), the light quantity can be ideally corrected by driving current at the end portion (100% electric current), but the light quantity is less ideally corrected at the central portion (such as 80% electric current) than at the end portion.